vni PROTECTIVE COLOUR OF SLOTH 171 as hooks to keep them suspended from the lower side of a branch. The three-toed sloth, Uradypus (or "Ai"), has the exceptional number of nine cervical vertebrae ; the two-toed sloth, Ckoloepus hojfmanni (or "Unau"), has the equally exceptional number of six. The hair is long and shaggy, and gets an adventitious green colour from the presence of minute algae.1 This gives to the animal the appearance of a lichen-covered bough, a resemblance which is increased in one species by an oval mark upon the back, which suggests forcibly a broken end of such a branch. The likeness of a Sloth to its surroundings is pointed FIG. 97.—Skull of Three-toed Sloth. Bradyptts tndacti/ftis. Lateral view, fr, Frontal ; j-n, jugal ; Icr, lachrymal j -max, maxilla ; <najt9 nasal ; pm\ parietal ; s.oc, supra- occipital ; ty, tympanic. (From Parker and Haswell's Zoology,) out by Z)r. Siernann,2 who observed that a species occurring in Nicaragua " has almost exactly the same greyish-green colour as Tillandsia usncoides, the so-called 'Vegetable Horsehair* common in the district. . . . If it could be shown that it frequented trees covered with that plant . . . there would be a curious case of mimicry between the sloth's hair and the Tillandsia, and a good reason why so few of these Sloths are seen." The stomach in the Sloths is complicated in structure, with several chambers ; one of these gives off a long crescent-shaped caecum- The skull of the Sloths agrees in a number of particulars with that of the Anteaters. 3 Tlio colour fad us in captivity owing to the disappearance of tlie algae. a Iu a letter addressed to Dr. Gray, quoted l>y the la-ttm* in a revision of the Sloths, /Voc. ZooL Soc. 1871, p. 428.